The invention relates to a tooth-root implant of high fatigue strength which is anchored in the jawbone and which has the ability of receiving a superstructure.
Natural teeth are connected to the jawbone by means of the parodontium. Because of the elasticity of the fibrous structure of the tooth-holding member, the tooth is not anchored rigidly in the jawbone, but is suspended elastically. This so-called physiological movability of the teeth varies from species to species. In humans, it amounts to approximately 30 .mu.m, that is to say under maximum chewing pressure the tooth is pressed approximately 30 .mu.m into the socket. At the same time, because of the special arrangement of the fibers in the parodontium, most of the jawbone surrounding the tooth socket is subjected not to compressive stress, but to tensile stress. The tooth-root implants which have become known hitherto cannot simulate the function of the parodontium to a sufficient extent. Since tooth-root implants are used, in many cases, for anchoring bridges, the second pier of which is still a natural tooth, it is necessary not to fasten the superstructure rigidly to the root implant, but to ensure the mobility between the superstructure and implant which a natural tooth possesses in relation to the jawbone.
Non-physiological moments of force occuring during the chewing process at the interface between the implant and bone bed are avoided in this way. A movable forcetransmitting structure between the superstructure and implant must also be designed so that load peaks during the chewing process are absorbed and transferred to the bed only in a damped state, because, in a natural tooth, the parodontium also performs the function of a "shock absorber".
A device for absorbing such pressure peaks is known from German Offenlegungsschrift 2,733,394. This describes a damping element in which a pin connected to the core of the tooth root projects into a metal sleeve with elastic plastic.
There are also known superstructures in which a crown is fastened elastically to the root in such a way that a cavity filled with elastomer is formed between a plug connected to the root and the crown; see German Offenlegungsschrift 2,247,649. A shock-absorbing arrangement which does without the insertion of elastic intermediate pieces or layers is described in German Offenlegungsschrift 2,830,025. Here, damping is brought about by a tapping piece receiving the superstructure and having a tilting support which terminates in the form of a lip and which rests sealingly on the outer edge of an intermediate piece.
A further damping member made of physiologically harmless polymers with a modulus of elasticity of between 1,000 and 5000 N/mm.sup.2 is described in German Offenlegungsschrift 2,824,214. The damping members which have existed or become known hitherto consist essentially of plastics, such as, for example, polyacrylate, polypropylene, polysultone or polymethacrylate; see German Offenlegungsschrift 2,419,080, German Offenlegungsschrift 3,043,336, German Auslegesschrift 2,413,883, German Offenlengungsschrift 2,704,390 and German Offenlegungsschrift 2,824,118.
In general, although plastics initially posses a modulus of elasticity suitable for damping purposes, nevertheless this and other mechanical properties undergo a detrimental change as a result of the continuous stress exerted during the chewing process. Plastics experience fatigue very quickly after a certain number of load alternations, so that the damping elements have to be exchanged even after only relatively short periods of time or after a small number of load alternations. U.S. Pat. No. 3,722,094 allows for this disadvantage by inserting a helical spring as a damping element between the superstructure and tooth root. However, an essential disadvantage of the arrangement, in addition to its constructive inadequacies, is that, under pressure load, a gap can form between the superstructure and the tooth root implant. Saliva and food residues can then penetrate into the cavity receiving the helical spring. Serious medical and hygienic problems arise as a result.